In wind farms, the electrical power produced by the wind energy installations is normally passed to a substation via a medium-voltage network at a voltage of, for example, 20 kV. To be transmitted to the external network, which is generally a high-voltage network, the electrical power is changed to a voltage of, for example, 110 kV, by means of a transformer. If the voltage in the high-voltage network changes, the voltage in the medium-voltage network can thus be kept constant by varying the step-up ratio between the high voltage and the medium voltage. This is known in the prior art.
As is known, wind energy installations for electricity generation are dependent on the capability to emit the electrical power at a specific voltage to the wind-farm-internal network and in this case the voltage may differ only slightly from the rated voltage. If the voltage is below the rated voltage, a higher current level is necessary in order nevertheless to be able to emit the rated power. Higher current levels can lead to an increased load on electrical components of the wind energy installation, such as the generator, cables, transformer etc. In order to prevent overloading, the wattless component is normally first of all limited in a situation such as this, before the real power is limited. In contrast, if the voltage in the wind-farm-internal network is above the rated voltage, then this can likewise lead to overloading of components of the wind energy installation, for example by the converter being overloaded. The voltage must therefore be within a narrow band above and below the rated voltage in order that the wind energy installation can emit the rated power and can at the same time comply with the wattless component requirements. Even if the wind energy installation is still operated within the permissible range, the load on the wind energy installation rises when operation is in the vicinity of the system limits. Operation at the edge of the system limits leads to a reduced wind energy installation life, and to increasing maintenance effort.